Choosing a Training Path:
Post Residency Training
In this pathway, the trainee completes the residency program and then embarks on a formal research training program, often leading to a PhD.
This route usually involves research training during a research fellowship. The major advantage of this route is the proximity of the research training to the start of independent practice, allowing the trainee to "hit the ground running" at the end of training. These programs may also provide for research funding for a few years while starting an independent faculty position. 
Some trainees elect to do research training outside a formal graduate program in a research lab. The primary drawback of post residency training, especially for those without a prior research background, is that it often means a shorter duration of formal training in research, as the personal and financial disincentives towards spending extra years pursuing a PhD vs. beginning clinical practice become an issue for some. Depending on the quality of the post-residency research experience, this route may not necessarily be disadvantageous; though for some, it may make competition for initial grants more difficult, at least in the short term, since their CV may not reflect enough high quality work. Biomedical research is highly competitive, so taking up your first faculty position with a strong CV is of great benefit.